The present invention relates to an iron oxide coloring agent for resins and, more particularly, to an iron oxide coloring agent which is capable of suppressing to the minimum the aging of a resin, especially the aging of a resin having chlorine atom and being colored with an iron oxide pigment which is caused by the influence of the heat applied to the resin during processing or the light during exposure to the open air.
Resins having chlorine atom, especially, chlorine-containing thermoplastic resins are used as molding materials. For example, polyvinyl chloride, chlorinated polyethylene, chlorinated polypropylene and polyvinylidene chloride are well known. Among these, polyvinyl chloride is widely used. The phenomenon of aging of chlorine-containing resins will now be explained citing a polyvinyl chloride representative of these resins.
In polyvinyl chloride, even if it is not colored with an iron oxide pigment, the polyvinyl chloride is decomposed and degenerated by heat-treatment during processing or light exposed to after it is molded into products, thereby giving rise to the phenomena of aging.
The phenomenon of aging in resins is caused by reason that a part of chlorine bonding in a polyvinyl chloride is decomposed at 100 to 200.degree. C. by heat or light action, thereby generating hydrogen chloride and forming a polyene structure having a double bonding, and further the hydrogen chloride generated secondarily acts on the resin itself, thereby cutting a part of C--C bonding of a polymer or producing a cross-linking as a chain reaction.
It is generally known that in order to suppress such aging, a neutralizer such as a lead compound and metallic soap for neutralizing the hydrogen chloride generated by decomposition or a resin stabilizer such as an organic tin compound and an epoxy compound which is effective in suppressing the generation of a double bonding, is added to a polyvinyl chloride powder for an ordinary thermoforming material even when it is not colored by an iron oxide pigment.
As coloring agents for polyvinyl chloride, inorganic pigments which are excellent in the light resistance and heat resistance, namely, iron oxide pigments such as ferric oxide pigments are conventionally used widely. However, since metals such as Mn, Fe, Co, Ni, Cu and Zn have an accelerating action of the dehydrochlorination, as well known, in the case of mixing an iron oxide pigment with a polyvinyl chloride as a coloring agent, the aging of the resin is greatly accelerated.
Various methods of improving the capability of iron oxide pigments to suppress aging of resins have been studied. For example, a method of improving an aging resistance by forming a continuous fine silica film on the surfaces of the particles of an iron oxide pigment (Japanese Patent Publication No. 54-7292 (1979)), a method of improving an aging resistance by coating the surfaces of the particles of an iron oxide pigment with glassy Na and/or K-Si (Japanese Patent Publication No. 53-11537 (1978)) and a method of improving an aging resistance by depositing SiO.sub.2 on the surfaces of the particles of an iron oxide pigment and further depositing aluminum hydroxide on the deposited SiO.sub.2 (Japanese Patent Application Laid-Open (KOKAI) No. 53-36538 (1978)) are known.
An iron oxide coloring agent having an excellent capability to suppress aging of resins is now in demand, but the above-described known iron oxide pigments cannot be said to impart a sufficient aging resistance, we will be shown in the later-described comparative examples.
Accordingly, it is the strongest demand to provide an iron oxide coloring agent excellent in imparting aging resistance.
As a result of the present inventors' studies, it has been found that by adding silicon dioxide fine particles or hydrated alumina fine particles and a silicate or an aluminate to a suspension containing plate-like hematite particles and adjusting the pH of the resultant suspension, the obtained plate-like hematite particles the surfaces of which are coated with amorphous silicon dioxide or hydrated alumina containing silicon dioxide fine particles or hydrated alumina fine particles having an average particle diameter of 50 to 1,000 .ANG., are excellent in imparting aging resistance and are useful as an iron oxide coloring agent. The present invention has been achieved on the basis of this finding.